Spectacle lenses for the correction of blurred vision of distant objects conventionally employ two surfaces of spherical form when the eye is free of astigmatism, or one surface of spherical form and the other of toroidal form when astigmatism requiring correction is present. For a given value of back vertex power, the curvatures of the surfaces comprising a lens of this type are fixed by the designer using a predetermined standard of optical performance for vision through the peripheral areas of the lens ("off-axis performance"). Such lenses are termed "best form" lenses.
Conventional best form lenses tend to be bulky and cosmetically unattractive in the higher powers. Table 1 illustrates the point for typical best form hard-resin lenses (refractive index n=1.498) having back vertex powers of +5.00 D and -5.00 D. The curvatures of the spherical surfaces of these lenses have been chosen, in the absence of chromatic aberration, to minimize the sum .vertline.MOE.vertline.+.vertline.OAE.vertline., where MOE is the mean oblique error [arithmetic mean of the tangential (T) and saggital (S) errors] and OAE is the oblique astigmatic error (T -S).
TABLE I ______________________________________ CHARACTERISTICS OF BEST FORM LENSES OF REFRACTIVE INDEX 1.498 AND 71 MM DIAMETER. CENTER OF ROTATION DISTANCE = 28.5 MM. OBJECT DISTANCE = 10,000 M. 1.53 power Edge Center Flat plate Back vertex of front thickness thickness thickness power (D) surface.sup.(1) (D) (mm) (mm) (mm) ______________________________________ +5.00 9.80 1.0 8.2 14.3 5.00 3.90 9.8 2.2 14.5 ______________________________________ .sup.(1) 1.53 surface power is an industry standard defined by the expression (n - 1)/r, where n = 1.53 and r is radius of curvature (in meters)
The table suggests that the +5.00 D lens will be bulbous in appearance owing to the relative strong front surface power (i.e., short radius of curvature), and the -5.00 lens will seem massive owning to its relatively thick edge. Both lenses exhibit a large flat plate thickness. (Flat plate thickness is the separation of two flat plates held against opposite sides of the lens.)
It is well known that the cosmetic appearance of the best form lenses can be improved by employing an aspherical surface of appropriate form in conjunction with a conventionally formed spherical or toroidal second surface. Davis (U.S. Pat. No. 3,960,442) discloses the basic concept of combining an aspheric surface with base curve selection to obtain desired design characteristics while maintaining optical performance. A specific lens series of the aspheric type described by Jalie (U.S. Pat. No. 4,289,387) utilizes a convex hyperboloidal front surface for plus power lenses, with the prescription to be formed on the back surface, and a concave hyperboloidal back surface for minus power lenses, with the prescription to be formed on the front surface. The use of the hyperboloidal surfaces provides lenses having reduced edge, center and flat plate thicknesses relative to those of their conventional best form counterparts. Moreover, by an appropriate choice of the "conic constant" associated with the hyperboloidal surface, the off-axis performance of the aspherical lens series can be made comparable to that of the conventional best form series.
The use of a concave hyperboloidal surface for minus power lenses presents a problem for prescription lens processing laboratories in that most laboratories lack the machinery necessary to generate, grind and polish the convex front surfaces of prescription lenses. The processing problem is resolved by incorporating a convex oblate ellipsoid on the front side of the lens, rather than a concave hyperboloid on the back side. Unfortunately, although the off-axis performance of such a lens can be adjusted by varying the conic constant of the oblate ellipsoid, the performance is not, in general, as good as that of the lens with concave hyperboloidal back surface.